Method of making grid resistors



Feb. 3, 1938. c)' M. OTTE METHOD OF MAKING GRID RESISTQRS 1935 2 Sheets-Sheet 1 Filed March 20,

Feb. 1, 1938. o. M. OTTE ETHOD OF MAKING GRID RESISTORS Filed March 2Q, 1955 2 Sheets-Sheet 2 Patented Feb. I, 1938 UNITED STATES ZJQ'LES'T PATENT OFFlCE legheny Steel Company,

Pennsylvania a corporation of Application March 20, 1935, Serial No. 12,011

6 Claims.

This invention relates to current limiting resistors, such as used for example in the starting circuit of an electric motor, and more particularly to a new and improved type of grid resistor and to a method of making the same.

An object of this invention is the provision of an improved grid resistor.

Another object of the invention is the provi sion of a method of making grid resistors from resistant strip material.

A further object of the invention is the provision of a method of making resistor grids from strip material. Without scrap loss and without causing any substantial variation in the resistance thereof at any point in its length.

A still further object of the invention is the provision of a grid resistor that shall have a high space factor, whereby for a given space in which gridresistors may be mounted, more resistance per grid and per group of grids may be obtained.

Other objects of the invention will in part be apparent, and will in part be obvious from the following description taken in conjunction with the accompanying drawings, in which:

Figure 1 is a View in front elevation of a grid resistor representing an embodiment of this invention;

Fig. 2 is a vertical edge view of the resistor;

Fig.3 is a top plan View of a strip of resistor material from which a resistor grid such as shown in Figs. 1 and 2 may be made in accordance with the method of this invention;

Figs. 4, 5, and 6 are fragmentary top plan, edge, and top plan views, respectively, of a portion of the resistor strip, showing successive steps employed in the development of the resistor grid of Figs. 1 and 2;

Figs. 7, 8, and 9 are enlarged fragmentary top plan, edge, and top plan views, respectively, of a portion of the resistor strip showing steps similar to those depicted by Figs. 4, 5, and 6, but modified to accommodate the method to Wider strip material;

' Fig. 10 is a fragmentary front view in elevation of a modified form of resistor made in accordance with the method of this invention;

Fig. 11 is a fragmentary view in front elevation of another form of resistor made in accordance with the method of this invention; and

Fig. 12 is a fragmentary view in front elevation of a prior art resistor, the prior art resistor being disposed between Figs. 10 and 11 and having lines superposed thereon showing the average length of current path of each of the grid resistors disclosed for purposes of comparison.

Throughout the drawings and the specification, like reference characters indicate like parts.

In many industrial plants and in mines, for example, where electric locomotives are employed or where electric cranes are used, it is sometimes difiicult to find sufficient mounting space for the grid resistors which will permit the grid resistors to clear walls, posts or pillars and to allow the passage of workers between the resistor frames and such walls, posts, or pillars. In many cases, the available space for mounting the grid resistors is so small that it is difiicult to accommodate the bulk of the grid resistors that is required by the motors with which they are used. Many attempts have been made to make resistors of such shape and form that the required resistor capacity in a given case will occupy only'a relatively small space. In other words, the trend has been for manufacturers of grid resistors to improve the space factor, that is, the ratio of resistance in ohms (the proper current carrying capacity of the resistors being assumed) to the volume of the space occupied thereby.

As will be apparent hereinafter, resistors made in accordance with the method of this invention will make possible an increase of from fifty to ninety percent in resistance per grid and a corresponding increase in the space factor. That is, for a given mounting space resistor grids having from fifty to ninety percent more resistance per grid, but the same current carrying capacity per grid, may be mounted in that space.

In Fig. 12 of the drawings a prior art resistor grid 9 is shown which is made from a wide strip of resistance material and which has a relatively good space factor compared to the space factor of prior art grid resistors, say for example the cast grid type. Resistor I is made by punching slots 2 which extend from opposite edges and transversely of the strip and in staggered relation so as to provide a grid comprising parallel return bends or legs 3. The ends of these return bends are punched as at 4 to receive the mounting bolts 5 and 6 of the support frame (not shown) on which a plurality of grids are supported and which make up the grid resistor unit. The resistor grids are insulated from the support bolts by tubes 5' and 6' which are slipped on over the bolts. In this form of resistor unit, bolts or rods 5 are of relatively large diameter and these constitute the main supports for the grid resistor unit. Bolts 6 which are of smaller diameter serve also to support the resistor grid unit, but their primary function is to hold the return bends of the resistor in place. In a resistor of this type the average length of the current path is indicated by broken line 7.

In making a resistor of this type much material is wasted as is apparent by inspection of Fig. 12. That is, all of the material which is punched out to form the holes through which the insulated rods or bolts pass and to form the return bend loops or legs, represents Waste material. It is not uncommon in the manufacture of grid resistors such as disclosed by Fig. 12, that as much as 43 percent of the material is wasted. Grid resistors of this type are usually made from alloy material which is expensive and where so much material is wasted, the cost of producing such resistor grids is greatly affected.

In accordance with the present invention no material is wasted so that the ratio of gross weight of material employed to the net weight of grid produced is, for all practical purposes, equal to unity. Also, the material is more efficiently utilized so that a greater average length of current path may be realized which means higher resistance per grid.

In Fig. 1 of the drawings a resistor 8 is shown which is designed to fit exactly the mountings of the prior art grid resistors shown in Fig. 12, and for that reason the support bolt apertures or holes are shown to be in the same positions and of proportionate size (note-the resistor of Fig. 1 is drawn to larger scale than the resistor of Fig. 12).

The resistor grid 8 is made from strip material having the necessary cross section required to carry the current which it must handle in any given case. Transverse sections of this material have relatively large major axes and relatively small minor axes, but the material is bent edgewise, that is about its major axis, at predetermined points to form a plurality of return bends having substantially parallel legs ii, the major axes of which are in substantially the same plane. In making this resistor, the material is so manipulated that it may be bent edgewise at the places where the bends are formed, while at the same time maintaining the same current conducting area or section throughout the full length of the resistor.

The alloys from which grid resistors are made have physical properties which make it difficult to edgewise bend such material when in strip form, without fracturing the same.

In accordance with this invention, the major axes of the strip at the points where the bends are to be made are reduced without changing in any material way, the cross section at these points. By reducing the major axes of the material at these points, the section moduli at these points are reduced and therefore the maximum fiber stresses are reduced. The maximum fiber stresses can be more or less limited to values far below the ultimate strength of the material by maintaining the proper relation between the radii of the bends and the major axes of the material at these bends.

In a preferred form of this invention, the major axes of the material are reduced at the points of bending by first slitting the strip longitudinally at spaced points or sections as at I and H, thereby forming narrow ribbons l2 and I3 on each side of the slits. The distance between slits and the length of these slits are determined by the length of the return bend loops, or the distance between the mounting rods and the radii of the bends. After slitting the material this way, the ribbons l2 and 13 are deformed and overlapped as shown in Fig. 6 to reduce the major axes and the section moduli of the strip at the places where the bends. are to be made. This operation may be performed by bending one ribbon above or partially above the plane of the sheet or strip and the opposite ribbon below or partially below the plane of the strip, see Fig. 5. After the material has been formed as in Fig. 5, the ribbons may be gripped in a suitable die and squeezed toward the center of the strip until they overlap at the middle portions thereof and only partially overlap from the middle towards the opposite ends thereof (see Fig. 6). The major axes of the strips are therefore, as shown, reduced approximately fifty percentum at these sections.

Having thus completed the preliminary steps in the forming of the resistor, the next steps in the formation thereof involve the edgewise bending of the strip at the points where the major axes have been reduced. The bending may be done on mandrels of the proper diameter. Since a certain amount of elongation takes place at the outer fibers of the bends, it is preferred to make the first bend at the middle of the strip and to then work towards the ends, for by so doing the elongation will be equally distributed throughout the length of the strip.

To make resistor grids that will fit the frame bolts on which the resistor grid of Fig. 12 is mounted, it is necessary to vary the distance between slits Hi and H because the vertical distance between the centers of the large bolts 5 is less than the vertical distance between the small bolts 6, and to make the slits of the sections which are bent to the radii of the insulating tube 5' for the large bolts, longer than those which are bent to the radii of the insulating tubes 6' for the small bolts. Because of the arrangement of the large and small bolts, the longer slits ID are located at the center and the opposite ends of the strip, and the shorter slits H are located between them.

After having slitted the strip and reduced the major axes thereof at the places where the bends are to be made, the bending of the strips to form the return bend loops is done. The first bend is made at the center of the strip about a mandrel having a diameter D as indicated in Fig. 1; the sections on each side of the middle are bent about mandrels having a diameter DI as indicated in Fig. l; and the ends of the strip are bent about a mandrel or mandrels having a diameter D. The bends on each side of the middle of the resistor strip are made progressively from the middle to the outer ends thereof, so that the elongation above mentioned will be equally distributed between the two halves of the resistor grid.

A resistor such as shown in Fig. 1, having the same dimensions X and Y as the resistor of Fig. 12, has a current carrying section at any point along its length which is uniform and for all practical purposes equal, but has a longer mean or average current path. The average or mean length of current path of resistor 8 is indicated by broken line H: and he average or mean length of path of resistor I (Fig. 12) is indicated by broken line I'B. A visual comparison of these paths shows that the resistors of Fig. 1 has about 48.25 percent longer path than the resistor of Fig. 12 and therefore, it has about 48.25 percent more resistance than the resistor of Fig. 12, but

both occupy the same amount of space. It is therefore apparent that a resistor grid such as shown in Fig. l and having the same resistance as the resistor in Fig. 12 will occupy less space than the latter, and that therefore, the former has the higher space factor.

By utilizing more insulated supporting bolts, of the same or different diameter, resistor grids may be made in accordance with this invention that will have the same overall dimensions X and Y with about 91 percent more resistance. Such a resistor is shown in Fig. 11 and is numbered l'l. This resistor has three relatively long legs l3, l9, and 29, the middle leg it! being double, and two relatively short double legs 2i and 22 interposed between the longer legs. The insulated support bolts 23 and 24 are grouped in parallel vertically spaced planes, with the larger bolts 24 at the four corners of the resistor grid. The bends in this resistor are made in accordance with the procedure described in connection with Figs. 1 to 6, inclusive, except that the end bends are turned in instead of out as in Fig. 1 in order to keep all parts of the resistor within the boundary of dimensions X and Y.

The increase in resistance which a resistor such as indicated in Fig. 11 aiiords over the resistor of Fig. 12 is readily apparent by comparison of lengths of the mean current paths thereof, broken line 25 indicating the mean current path of resistor of Fig. 11, and broken line i indicating the mean current path of the resistor of Fig. 12.

By increasing one of the overall dimensions of the space occupied by the resistor of Fig. 12, say dimension X, the number of full length loops of a resistor made in accordance with this invention may be increased and with a corresponding increase in resistance per grid, and all this without materially increasing the overall dimensions thereof. Fig. 10 shows a resistor 26 of this type, its dimension X, for example, being shown as being approximately twenty percent greater than the same dimension of the resistor of Fig. 12, but its dimensions Y is the same. The resistance of the grid of Fig, 11 however is approximately 101 percent higher than that of the grid of Fig. 12. This increase is shown by the relative lengths of the average or mean current paths 2? and 1, respectively, of these two resistors.

In order that a ready comparison in regard to the values of resistance per grid, may be made between the resistor grids of Figs. 1, 2, 10, and 11, and the grid of Fig. 12, the lengths of average or mean current paths of these resistors are superimposed on Fig. 12 and identified in accordance with the legends thereto appended.

The method of producing resistors as depicted by Figs. 3, 4, 5, and 6 applies more particularly to the making of resistor grids from relatively narrow strip material, but where wider strip material is used, the method may be modified in accordance with the procedure depicted by Figs. 7, 8, and 9. The strip material shown in Fig. '7 is approximately one-half wider than the strip shown in Fig. 3 and in order to reduce its section moduli about the major axes thereof, at the points where the bends are to be made, the strip is slit longitudinally, say twice at each section, thereby dividing the strip into three narrow ribbons, which, when superimposed one on the other as indicated, the strip may be bent edgewise to a relatively sharp radius without rupturing the material at the points where the maximum fiber stresses occur. Thus, it is apparent that the number of slits which are made in the material will depend largely upon 'the Width of the strip and on the radii of the bends.

By comparing the'grid resistors disclosed as embodying'this invention to the grid resistor of Fig. 12, it will appear that the latter has more heat dissipating surface in the regions of the support bolts or rods. However, if this is a deficiency in the former, the rate of heat dissipation of the resistors of this invention may be adequately increased at the bends by mounting large washers for example on the support bolts on either or both sides of the grid but in contact therewith, in whichcas'e insulation would be placed between adjacent washers for reasons well understood by those skilled in this art.

Although a convenient procedure has been disclosed whereby the section moduli with respect to the major axes of the material may be reduced to the amount required for bending edgewise to a given radius, it will be apparent that the section moduli may be reduced in other ways without changing the current conducting section of the strip or without departing from the spirit or scope of the invention.

In some cases it may be necessary to stretch the grid resistors made in accordance with this invention, in a direction lengthwise of the loop legs in order to accurately size the grids and make them fit the frame bolts.

While several forms of the invention have been shown, each having the same-with the exception of resistor grid 26over all dimensions as the prior art resistor of Fig. 12, it is to be understood that the relative dimensions and proportions have been given merely for the purpose of accentuating the advantages of and the results that may be gained from this invention.

What I claim as new and desire to secure by Letters Patent is:

1. Method of making resistor grids of the return bend loop type from strip material which consists in slitting the strip lengthwise at predetermined points in the length thereof to form narrow ribbon portions, deforming strip at the slitted portions so as to cause the ribbon portions on opposite sides of the slit to overlap, and bending the strip edgewise at the overlapped portions so as to form the return bend loops of the grid.

2. The method of making resistor grids of the character described, from strip material which consists in subdividing lengthwise spaced sections of the strip into ribbons which are unitary at their ends with the strip, displacing the ribbons transversely of the strip to at least partially overlap the same, and then bending the strip edgewise at said ribboned sections to form the return bends of the resistor grid.

3. The method of making resistor grids, of the character described, from strip material which consists in subdividing lengthwise spaced sections of the strip into ribbons of substantially equal width which are unitary at their ends with the strip, displacing the ribbons transversely of the strip to at least partially overlap the same, and then bending the strip edgewise at said ribboned sections to form the return bonds of the resistor grid.

4. A method of making a resistor grid having a substantially constant resistance throughout its length which comprises reducing the Width of a strip of suitable material at spaced points along its length without reducing the current conducting section of the strip material at such points and bending the strip edgewise at such spaced points to form the return bend loops of the grid.

Lil

5. A method of making a resistor grid having a substantially constant resistance throughout its length without the production of scrap which comprises subjecting a strip of suitable material to such operations as will cause partial overlapping of the material of the strip at spaced points thereof Without removing material and Without reducing the current conducting section of the strip material at such points and bending the strip edgewise at such overlapping points in such manner as to form the return bend loops of the grid.

6. A method of making a resistor grid having a substantially constant resistance throughout its length which comprises reducing the width of a suitable flat strip of resistance material and proportionately increasing the thickness thereof at spaced points along the strip, thereby maintaining the current conducting section of the strip uniform, edgebending the strip at the points of reduced Width and increased thickness in such manner as to distribute equally any elongation resulting from such bending and then adjusting 10 the loops to size.

OTHO M. OTTE. 

